Contract level checker: Disallow free function redefinition and alias

shadowing another free function

Co-authored-by: chriseth <chris@ethereum.org>

test/libsolidity/syntaxTests/multiSource/two_imports_same_function.sol

@@ -0,0 +1,9 @@
+==== Source: s1.sol ====
+function f() pure returns (uint) { return 1337; }
+==== Source: s2.sol ====
+import {f as f, f as f} from "s1.sol";
+contract C {
+  function g() public pure returns (uint) {
+    return f();
+  }
+}